1. Field of the Invention
The invention relates to an exhaust gas retreatment system of an internal combustion engine. The invention furthermore relates to a method for operating such an exhaust gas retreatment system.
2. Description of the Related Art
From practice, exhaust gas retreatment systems of internal combustion engines are known. Such systems comprise a particle filter, at least one exhaust gas retreatment assembly, which is arranged, with respect to the flow direction of the exhaust gas, upstream of the particle filter, and, if appropriate, an exhaust gas retreatment assembly positioned, seen in the flow direction of the exhaust gas, downstream of the particle filter. An exhaust gas retreatment assembly which, in the flow direction of the exhaust gas, is positioned upstream of the particle filter is in particular an oxidation catalytic converter. An exhaust gas retreatment assembly, which is positioned downstream of the particle filter, can be an SCR-catalytic converter and/or a silencer and/or a heat exchanger and/or a desulphurisation assembly. The term particle filter is used to mean both conventional particle filters, through which exhaust gas flows, and also particle separators, in the case of which the exhaust gas flow is conducted along a separating structure.
In particular when, seen in flow direction of the exhaust gas, an oxidation catalytic converter is positioned upstream of the particle filter, nitrogen monoxide (NO) in the exhaust gas is oxidized into nitrogen dioxide (NO2) in the oxidation catalytic converter with the help of the residual oxygen (O2) contained in the exhaust gas flow according to the following equation:2NO+O2→2NO2 
During this oxidation of nitrogen monoxide into nitrogen dioxide the equilibrium of the oxidation reaction at high temperatures is on the side of nitrogen monoxide. As a result the achievable component of nitrogen dioxide is greatly limited at high temperatures.
In the particle filter, the nitrogen dioxide extracted in the oxidation catalytic converter is converted with carbon-containing particles collecting in the particle filter, so called soot, into carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2) and nitrogen monoxide (NO). Here, in the sense of a passive regeneration of the particle filter, a continuous removal of the carbonaceous fine material particles or of the soot deposited in the particle filter takes place, this conversion taking place according to the following equations:2NO2+C→2NO+CO2 NO2+C→NO+CO2C+2NO2→N2+2CO2 
In particular when, with such a passive regeneration of the particle filter, no complete conversion of the carbonaceous fine material particles or of the soot deposited in the particle filter can take place, the carbon component or soot component in the particle filter increases, wherein the particle filter then has a tendency towards clogging, as a result of which ultimately the exhaust gas backpressure at an internal combustion engine situated upstream of the exhaust gas retreatment system increases.
A rising exhaust gas backpressure at the internal combustion engine reduces the power of the internal combustion engine and causes increased fuel consumption.
In order to avoid an increase of the quantity of carbonaceous fine material particles or of the soot in the particle filter and thus a clogging of the same, it is already known from practice to provide particle filters with a catalytic coating. Here, platinum-containing coatings are preferentially employed. The use of such particle filters with catalytic coating, however, can only insufficiently prevent the charging of the particle filter with carbonaceous fine material particles, i.e., with soot, at low exhaust gas temperatures.
If the soot charge of the particle filter exceeds a certain limit, which depending on the filter material is between 3 and 10 g of soot/l of filter substrate, uncontrolled ignition of the soot can occur. In this case, the soot combusts suddenly, which because of the exothermic of the carbon oxidation results in a temperature increase of above 1000° C., which results in a thermal damaging of the filter material.
Furthermore, employing active regeneration of the particle filter is known from practice for reducing the charging of a particle filter with soot. In the case of such an active regeneration of the particle filter, the exhaust gas temperature is periodically actively raised to 500° C. to 650° C., for example through the addition of fuel to the exhaust gas flow, in order to burn off carbonaceous fine material particles or soot particles which have accumulated in the particle filter by way of an exothermic reaction or oxidation of the hydrocarbons. The burning-off of the carbon with the help of oxygen in a particle filter in this case takes place according to the following equation:C+O2→CO2 
At the greatly elevated temperatures, a conversion of nitrogen monoxide can also be observed.2C+2NO→N2+2CO
In the case of an active regeneration of a particle filter by burning of the soot particles there is likewise the risk that soot charges of the particle filter are high enough that due to the exothermic carbon oxidation in the particle filter, a severe temperature increase of above 1000° C. develops. In the case of such a severe temperature increase damage to the particle filter and/or exhaust gas retreatment assemblies connected downstream can occur. As already explained, this is also problematic in the case of a passive regeneration of a particle. Here, too, a temperature increase of over 1000° C. in the particle filter can occur as a result of a severe exothermic reaction, as a result of which the particle filter and/or exhaust gas retreatment assemblies connected downstream of the same in turn are exposed to a damage hazard.
To date it is not possible, in particular when with a passive regeneration of a particle filter an uncontrolled burning-off of fine material particles or soot particles occurs in the particle filter, to detect this in time. Furthermore, it is not possible to date in particular when fine material particles or soot particles in terms of active regeneration burn off in a controlled or alternatively uncontrolled manner, to safely and reliably determine the degree of charging of the particle filter with soot prior to the soot burn-off and/or the temperature increase of the particle filter as a consequence of the soot burn-off and/or a speed of the soot burn-off.